This invention relates to a resist composition, typically chemically amplified positive resist composition, which has a remarkably increased contrast of alkali dissolution rate before and after exposure, a high sensitivity, high resolution, and etching resistance and lends itself to the microfabrication of VLSI""s.
While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. Deep-UV lithography is capable of achieving a minimum feature size of 0.5 xcexcm or less and, when a resist having low light absorption is used, can form patterns with sidewalls that are nearly perpendicular to the substrate.
Recently developed acid-catalyzed chemical amplification positive resists, such as those described in JP-B 2-27660, JP-A 63-27829, U.S. Pat. Nos. 4,491,628 and 5,310,619, utilize a high-intensity KrF excimer laser as the deep-UV light source. These resists, with their excellent properties such as high sensitivity, high resolution, and good dry etching resistance, are especially promising for deep-UV lithography.
Such chemically amplified positive resists include two-component systems comprising a base resin and a photoacid generator, and three-component systems comprising a base resin, a photoacid generator, and a dissolution inhibitor having acid labile groups.
For example, JP-A 62-115440 describes a resist material comprising poly-4-tert-butoxystyrene and a photoacid generator, and JP-A 3-223858 describes a similar two-component resist material comprising a resin bearing tert-butoxy groups within the molecule, in combination with a photoacid generator. JP-A 4-211258 describes a two-component resist material which is comprised of polyhydroxystyrene bearing methyl, isopropyl, tert-butyl, tetrahydropyranyl, and trimethylsilyl groups, together with a photoacid generator.
JP-A 6-100488 discloses a resist material comprised of a polydihydroxystyrene derivative, such as poly[3,4-bis(2-tetrahydropyranyloxy)styrene], poly[3,4-bis(tert-butoxy-carbonyloxy)styrene] or poly[3,5-bis(2-tetrahydropyranyloxy)styrene], and a photoacid generator.
The base resins in these resist materials bear acid labile groups on side chains. When these acid labile groups are groups such as tert-butyl and tert-butoxycarbonyl which are cleaved by strong acids, the resist pattern tends to take on a T-top profile. By contrast, when the acid labile groups are alkoxyalkyl groups such as ethoxyethyl which are cleaved by weak acids, the corresponding resins have the drawback that the pattern configuration is considerably narrowed as the time interval between exposure and heat treatment increases. Moreover, the presence of bulky groups on the side chains lowers the heat resistance of the resin and leads to an unsatisfactory sensitivity and resolution. These problems have hitherto prevented the practical implementation of either approach.
To provide higher transparency and firm adhesion to substrates and to improve footing on substrates and etching resistance, JP-A 3-275149 and 6-289608 propose resist materials using a copolymer of hydroxystyrene and tertiary alkyl (meth)acrylate. The resist materials of this type suffer from poor heat resistance and an indefinite pattern profile after exposure and are not satisfactory in etching resistance as well. Since the current progress toward higher resolution entails the simultaneous progress toward thinner film patterns, it would be desirable to have a resist material with higher etching resistance.
An object of the invention is to provide a resist composition, especially chemically amplified positive resist composition, which is superior to prior art resist compositions in sensitivity, resolution, exposure latitude and process flexibility, and has a satisfactory pattern profile after exposure and high etching resistance. Another object is to provide a patterning process using the resist composition.
It has been found that a polymer having recurring units of the following general formula (1) or (2) and having a weight average molecular weight of 1,000 to 500,000 is an effective base resin in a positive resist composition, especially chemically amplified positive resist composition. The chemically amplified positive resist composition containing a photoacid generator and an organic solvent as well as the polymer has many advantages including an increased dissolution contrast of a resist film, high resolution, improved latitude of exposure, improved process flexibility, a good pattern profile after exposure, and high etching resistance. The composition is thus suited for practical use and advantageously used in microfabrication, especially in VLSI manufacture.
In one aspect, the invention provides a resist composition comprising a polymer comprising recurring units of the following general formula (1) and recurring units having acid labile groups which units increase alkali solubility as a result of the acid labile groups being decomposed under the action of acid, and having a weight average molecular weight of 1,000 to 500,000. 
Herein R1 and R2 each are a hydrogen atom, hydroxyl group, substitutable hydroxyalkyl group, straight or branched alkyl group, substitutable alkoxy group, or halogen atom, and n is 0 or a positive integer of 1 to 4.
In another aspect, the invention provides a resist composition comprising a polymer comprising recurring units of the following general formula (2) and having a weight average molecular weight of 1,000 to 500,000. 
Herein R1, R2 and R4 each are a hydrogen atom, hydroxyl group, substitutable hydroxyalkyl group, straight or branched alkyl group, substitutable alkoxy group, or halogen atom, R3 and R5 each are hydrogen or methyl, R6 is a hydrogen atom, methyl group, alkoxycarbonyl group, cyano or halogen atom, R7 is an alkyl group of 1 to 20 carbon atoms, n is 0 or a positive integer of 1 to 4, m is 0 or a positive integer of 1 to 5, p, q and s each are 0 or a positive number, and r is a positive number.
In formula (2), one or both of R1 and R4 are preferably selected from among groups of the following formulae (3) and (4), straight, branched or cyclic tertiary alkoxy groups of 4 to 20 carbon atoms, trialkylsiloxy groups whose alkyl portions each have 1 to 6 carbon atoms, and oxoalkoxy groups of 4 to 20 carbon atoms. 
Herein R8, R9, R11 and R12 are independently selected from hydrogen and straight or branched alkyl groups of 1 to 8 carbon atoms, R10 is a monovalent hydrocarbon group of 1 to 18 carbon atoms which may be separated by an oxygen atom, or a pair of R8 and R9, R8 and R10, or R9 and R10 may form a ring and each of R8, R9 and R10 is a straight or branched alkylene group of 1 to 18 carbon atoms when they form a ring, R13 is a straight, branched or cyclic alkyl group of 4 to 40 carbon atoms, and xe2x80x9caxe2x80x9d is 0 or a positive integer of 1 to 5.
In a second aspect, the invention provides a chemically amplified positive resist composition comprising (A) an organic solvent, (B) the above-defined polymer as a base resin, and (C) a photoacid generator. The resist composition may further include (D) a dissolution inhibitor. Preferably the resist composition further includes (E) a basic compound.
In a third aspect, the invention provides a process for forming a resist pattern comprising the steps of applying the resist composition onto a substrate to form a coating; heat treating the coating and then exposing it to high-energy radiation or electron beams through a photo mask; and optionally heat treating the exposed coating and developing it with a developer.